The length of raw water gravity mains which convey the water to plant complex is about 15.0 km. When snow thaws at the higher altitudes, the turbidity level increases to 100 NTU for one to three months.

The treatment plant has one plant of 4.5 MLD capacity constructed in the year 1960 and another of 9.0 MLD capacity constructed in the year 1975. Both the plants are having sludge blanket type vertical flow settling tanks, followed by rapid sand gravity filters. The raw water (13.5mld) is initially measured over a weir and then is conveyed to a rectangular pre-settling tank. From presettling tank the water is conveyed to the treatment plants mentioned above. The entire complex is on a steeply sloping hill. The hydraulic gradient between the receiving chamber and pure water storage sumps is 22.00m. There is no access to treatment units by motorable roads. Power supply (single phase) though available at site is completely unreliable. The wash water pumps provided to fill the tanks are out of order due to lack of spares. The wash water tanks are presently filled up by tapping raw water at higher elevation and by conveying it to tanks by a pipeline. The treated water from sumps is conveyed to Gangtok town by gravity.

The supply is required to be augmented to 34 MLD due to increase in demand. The laying of additional water mains was completed in the year 1998-99. There is a shortfall of 20.5 MLD in the treatment plant capacity. The P.H/.E. dept. has decided to enhance the treatment capacity by utilizing the existing units wherever possible with minimum new civil construction.

The proposal for augmentation of treatment capacity in a nutshell is as follows. A circular cascade aerator of capacity 34 MLD will be constructed to receive the entire raw water flow. It will be followed by flow measuring channel (stilling chamber) will be an up-flow unit with detention time of 7-8 minutes. Split flow weirs will be located on the outlet side of the grit chamber (with isolation gates) to divide the flow into different parallel streams. Split flow weirs will also be used as rapid mix weirs for alum and lime dosing.

In this particular case, the stilling chamber is also made as grit chamber to avoid accumulation of sand and grit in flocculation zone (tank). Cascade Aerator is provided to oxidize trace iron (0.2-0.3 ppm). Lime dosing is incorporated as a system redundancy measure because sometimes in wet weather season, the raw water is tapped from the streams at lower elevation. The physio-chemical quality of this water is not known. This is a precautionary measure.

From grit chambers, one stream will be laid to the existing plant of 4.5 MLD capacities (1960). Another stram will convey water to 9 MLD plant (1975). No modifications are suggested to existing plants at present. The balance flow of 20.5 MLD will be split into streams and conveyed to existing pre-settling tank. It is proposed to convert this tank into two numbers of tube settling tanks with flocculation facilities. Each tank will be of size 17.8m x 9.35m x 3.50m swd.

For flocculation, two numbers of paddle type slow agitators mounted on a vertical shaft will be placed on a mild steel supporting structure. The drive unit will be “water turbine” powered by sacrificing some head between grit chambers and settling tank. The size of the flocculation compartment will be 9.35m x 8.3m (DT – 30-35 min). The outlet side of the pre-settling tank will be retrofitted with tube modules by constructing a partition wall and appropriate supporting system for modules and troughs. The size of this compartment will be 9.35m x 9.50m wit surface loading rate 4800 lph/sqm. It will not be possible to construct hoppers for removal of sludge due to limitation of tank depth. As the silt load is less and as most of the sludge will be in a form of very fine micaceus sand, manual cleaning of tanks once in six months should suffice.

This additional settled water of 20.5 MLD will be conveyed to the proposed filter house. The existing filter beds are old and will require extensive structural modifications to convert them into high rate filters. Therefore four new rapid sand gravity filter beds will be constructed with influent flow splitting weirs, without rate of flow controllers. The filtered water will be conveyed to the storage sump by laying additional pipeline. A new wash water tank will be constructed on the ground (not above the filter house) at an elevation 12-15m above that of the filter beds. This tank will use raw water tapped from still higher elevation (may be from the immediate break pressure sand filters (one working and one standby) are proposed on the incoming raw water main as a sort of roughing treatment. This is possible because of the excellent raw water quality for eight to ten months in a year.

Not a single pumping can be envisaged in the process or plant, as it will immediately affect the reliability of the system. The processes need to be independent of electrical power and as far as possible should be non-mechanical. This is a typical “hill” case, encountered in the north and north-eastern states of India.